The invention relates to a circuit arrangement for demodulating a frequency-modulated received signal into a demodulated output signal.
Such a circuit arrangement is disclosed in, for example, the German Offenlegungsschriften 3702855. It comprises a delay arrangement to which the received FM signal is applied and which delays it by a predetermined time delay, as well as an exclusive-OR circuit or a multiplier which combine the delayed and undelayed signals to derive demodulated signal. This demodulated signal is in the form of a pulsewidth modulated voltage, whose mean value can be recovered, free from harmonics, by a subsequent low-pass filter. The width of the pulses of the signal at the output of the exclusive-OR member and consequently the mean value at the output of the low-pass filter are linearly related to the frequency of the frequency-modulated signal. Changes in the frequency of the frequency-modulated signal, more specifically its modulation, consequently change the mean value and as a result thereof the signal supplied from the output of the low-pass filter, so that the frequency modulation is directly converted into an analog signal (for example an analog voltage).
For the case in which the carrier frequency of the frequency-modulated signal assumes a fixed, stable value, the signal at the output of the low-pass filter correctly represents the value of the frequency modulation of the carrier frequency of the frequency-modulated signal. If, in contrast thereto, the carrier frequency fluctuates, these fluctuations are transferred to the mean value and consequently distort the information content transmitted by the frequency-modulated signal. This may happen, for example, when the frequency-modulated signal is read from a record carrier which is mechanically moved relative to a reading device.
The invention has for its object to implement a circuit arrangement of the type defined in the opening paragraph such that fluctuations of the carrier frequency do not have any influence on the mean value of the demodulated signal and consequently on the information content to be recovered from the frequency-modulated signal.